A problem with conventional electronic lamps is that they often use an electronic circuit to regulate a constant current supplied to the light emitting devices (LEDs). This produces an incremental negative resistance as a load to the ballast they are connected to, however, a typical ballast cannot properly drive such a load. This condition causes a ballast to operate at some output level below its intended operating point or can cause a runaway condition that causes the ballast output voltage to increase to the point that can destroy the LED lamp components. Another problem with conventional electronic lamps is that they often use a transformer to match the ballast output voltage to the LED string voltage. This can be optimized to provide a load to the ballast that causes it to operate at its designed output level. However, the LED forward voltage changes due to operating temperature and aging effects. This change causes the optimized operating point to shift to a less efficient one because the effective load impedance the ballast sees changes. Another problem with conventional electronic lamps is that they often only connect to one electrode at both ends of the tube leaving one electrode at both ends of the tube unconnected. This can cause three problems when used with some ballasts.
First, it requires that the LED lamp be connected to an electrode in series with a capacitor. Such capacitors are typically in series with one of the two electrodes at either end of the tube in ballasts using voltage mode filament heating which results in a change in the load seen by the ballast and thus a shift in the operating point away from the optimum value. Second, in the case of rapid start ballasts that utilize current mode filament heating, the LED lamp is connected to an electrode which, in turn, is connected to a free floating capacitor inside the ballast, thereby providing no continuous electrical path in the lamp circuit and preventing the LED lamp from turning on. Third, modern electronic ballasts also have circuitry to detect broken lamp filaments or disconnected tubes by monitoring the filament resistance across these two terminals. An LED lamp that does not mimic the filament, when one electrode is completely disconnected, will cause these ballasts to shut down.
In these respects, the lamp ballast impedance controlled electronic lamp circuit according to the present invention substantially departs from the conventional concepts and designs of the prior art, and in so doing provides an apparatus primarily developed for the purpose of allowing a variety of electronic light emitting devices to be powered by a lamp ballast.